glossary: define commit-ish (a.k.a. committish)

Signed-off-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -82,6 +82,18 @@ to point at the new commit.
 	to the top <<def_directory,directory>> of the stored
 	revision.
 
+[[def_commit-ish]]commit-ish (also committish)::
+	A <<def_commit_object,commit object>> or an
+	<<def_object,object>> that can be recursively dereferenced to
+	a commit object.
+	The following are all commit-ishes:
+	a commit object,
+	a <<def_tag_object,tag object>> that points to a commit
+	object,
+	a tag object that points to a tag object that points to a
+	commit object,
+	etc.
+
 [[def_core_git]]core Git::
 	Fundamental data structures and utilities of Git. Exposes only limited
 	source code management tools.